The present invention relates to a process for removing carbon dioxide from combustion exhast gas, in particular, that uses an alkanolamine as the absorbent, and to an apparatus to be employed therefor.
In recent years, the so-called "greenhouse effect" due to increasing concentration of CO.sub.2, in the atmosphere has come to the forefront as an essential causal factor for the phenomenen of global climatic warming. Thus, it has become an international urgent theme to provide an effective countermeasure therefor, in order to realize protection of the global environmental condition. CO.sub.2 has many origins widespread over every field of human activity that uses combustion of fossile fuels. It is therefore an international trend to prescribe a more strict limitation on the amount of emission of CO.sub.2 into the atmosphere. Under these circumstances, there has been developing intensive research, in particular, in power plants which consume large amounts of consumption of fossile fuel, to achieve an effective technical measure of recovering CO.sub.2 by bringing combustion exhaust gas from boiler plants into contact with an absorbent, such as an aqueous solution of an alkanolamine etc., and to achieve a practical way for effecting storage of the recovered CO.sub.2 without exhausting it into the atmosphere.
Here, there may be employed as the absorbent for CO.sub.2, for example, aqueous solutions of alkanolamines, such as monoethanolamine, diethanolamine, triethanolamine, methyl diethanolamine, diisopropanolamine and diglycolamine as well as mixtures of these amines. Among them, usually an aqueous solution of monoethanolamine (abbreviated hereinafter as MEA) is employed preferably in a form of an aqueous solution.
One typical example of an apparatus for effecting the conventional process for CO.sub.2 removal using an aqueous solution of monoethanolamine (MEA) as the absorbent is explained now with reference to FIG. 4 appended herewith.
The apparatus shown in FIG. 4, which may be employed primarily for removing CO.sub.2 from the combustion exhaust gas from a fire source using a hydrogen-rich fuel, such as liquefied natural gas (LNG), consists essentially of a CO.sub.2 -removing tower 01, which comprises a lower packed section 02 for effecting an essential absorption of CO.sub.2 from the combustion exhaust gas by an MEA aqueous solution; an upper packed section 03 for effecting depletion of MEA content in the gas after having been treated in the lower packed section 02; an entrance 04 for the combustion exhaust gas supplied; an exit 05 of the final treated gas; a supply line 06 for the aqueous MEA solution; a first nozzle assembly 07 for spraying the MEA aqueous solution; a condensate accumulating tray 08, which may eventually be dispensed with, for receiving the condensate formed in the upper packed section 03; a circulation pump 09 for maintaining circulation of the condensate within the upper packed section; a heat exchanger 010 for effecting cooling of the condensate; a second nozzle assembly 011 for spraying the circulating condensate over the upper packed section 03; a discharge outlet 012 for discharging the spent aqueous MEA solution containing the absorbed CO.sub.2 out of the tower 01; and a feed blower 013 for boosting the combustion exhaust gas from a primary scrubbing stage into the absorption tower 01 via the entrance 04. The primary scrubbing stage consists essentially of a circulation system comprising a cooling section 015 for cooling the combustion exhaust gas supplied via a gas supply conduit 014 to effect condensing of the moisture content in the exhaust gas; a circulation pump 016 for circulating the condensate; a heat exchanger 017 for effecting heat exchange to cool the circulating condensate; and a nozzle assembly 018 for spraying the cooled condensate over the cooling section 015 to effect cooling and primary scrubbing of the combustion exhaust gas, and is provided with a condensate extracting line 019 for extracting the excessive amount of condensate out of the system.
The combustion exhaust gas from, for example, the boiler plant of a power station, having in general a temperature of 100-150.degree. C., is supplied first to the primary scrubbing stage, in which it is cooled in the cooling section 015 while forming a condensate which accumulates in its bottom and is employed as the cooling and scrubbing liquor sprayed from the nozzle assembly 018 under circulation by the circulation pump 016 with cooling by the heat exchanger 017, wherein a part of the condensate is continuously extracted out of the system via the extraction line 019. The combustion exhaust gas after having passed the primary scrubbing stage is supplied to the CO.sub.2 -removing tower 01 at the entrance 014 via a booster blower 013. The combustion exhaust gas supplied to the CO.sub.2 -removing tower is brought into contact with an aqueous MEA solution having a definite temperature and concentration supplied from the supply line 06 and sprayed from the first nozzle assembly 07 over the lower packed section 02 in counterflow to the rising gas, whereby the CO.sub.2 content in the combustion exhaust gas is removed by absorption by the aqueous MEA solution. The aqueous MEA solution containing thus the absorbed CO.sub.2 is discharged out of the CO.sub.2 -removing tower 01 via the outlet 012 and is then fed to a regeneration tower, not shown, for regenerating the spent aqueous MEA solution, from which the regenerated aqueous MEA solution is returned to the CO.sub.2 -removing tower 01 at the supply line 06.
On the other hand, the combustion exhaust gas which has been subjected to the CO.sub.2 removal in the lower packed section 02 flows up passing through a layer of the condensate accumulated on the condensate accumulation tray 08 into the upper packed section 03. The temperature of the gas entering the upper packed section 03 has been elevated by the exothermal reaction of absorption of CO.sub.2 with MEA effected in the lower packed section 02, so that the gas entering the upper packed section 03 has a higher content of vaporized MEA corresponding to the saturation concentration thereof in the gas at such elevated temperature. Therefore, the combustion exhaust gas which has thus been subjected to the CO.sub.2 removal should not be exhausted out to the atmosphere as such in consideration of the possible pollution of the atmosphere and the loss of MEA. Thus, the combustion exhaust gas having been denuded of its CO.sub.2 content in the lower packed section 02 is then treated in the upper packed section 03 in such a manner that a suitable amount of the condensate formed and separated in the upper packed section is sprayed over the upper packed section 03 through the second nozzle assembly 011 under circulation thereof by the circulation pump 09 through a cooling means (the heat exchanger 010) so as to effect contact of the cooled condensate with the rising gas in counterflow to each other to lower the temperature of the gas while at the same time to condense the water vapor together with MEA to decrease the MEA concentration in the gas, in order to prevent discharge of any harmful amount of MEA into the atmosphere.
The above example of the prior technique shown in FIG. 4 is employed primarily for firing stations using a fuel containing a substantial amount of hydrogen, such as LNG, capable of forming a sufficient amount of water vapor originated from the combustion of such a hydrogen-rich fuel for permitting use as the water source for building up the aqueous MEA solution and for cooling the combustion exhaust gas. The condensate thus formed is exhausted from the line 019 as excess water.
Alternatively, there has been proposed also a system in which the cooling of the combustion exhaust gas is effected using an externally supplied amount of water, which may be employed principally for boiler plants burning a fuel having high content of carbon, such as coal, heavy oil or so on. Here, it is, in general, unnecessary to provide the heat exchanger 017, as will be explained later on, but is only necessary to bring the combustion exhaust gas into contact with water held in circulation within the system to effect cooling of the combustion exhaust gas by the latent heat of vaporization of water. Here, an amount of fresh water should be supplied from outside, in order to replenish the amount of water lost gradually during the operation by giving off to the atmosphere by evaporation.
Though the prior art explained as above with reference to the appended FIG. 4 may be useful as such for a specific field of application, it possesses a disadvantage that the level of leakage of the absorbent, i.e. MEA, from the CO.sub.2 -removing tower into the atmosphere is still high, causing thereby a corresponding degree of atmospheric pollution and a larger loss of the expensive absorbent. In addition, it was recognized that ammonia was detected, though in a quite small amount, in the gas discharged out to the atmosphere from the exit 05 of the CO.sub.2 -removing tower 01 of the prior art apparatus. This ammonia may assumably be derived from a partial decomposition of MEA in the treatment system. It may also be possible for the reason therefor that the fuel would have had originally a residual ammonia content added in order to decrease the NO.sub.x level in the combustion exhaust gas. In any case, an occurrence of ammonia, though in a trifling amount, may be a further origin of unpleasant smell and bring about an emvironmental pollution when emitted to the atmosphere, so that it has to be removed from the gas before it is discharged out to the atmosphere. Here, it was a problem that the level of ammonia is quite low and, therefore it has to be removed at a sufficient efficiency.